GRAIL

Two spacecraft that will head for the Moon as early as Thursday have a unique mission ahead of them: They’ll play a three-month game of follow the leader.

Together, the two craft are known as GRAIL — Gravity Recovery and Interior Laboratory. They’ll map the Moon’s gravity in unprecedented detail. The information will help lunar scientists probe the structure and history of the Moon’s crust.

To do so, the instruments of the twin probes will track the relative positions of the two craft to within a few millionths of an inch. As they pass over regions with slightly different gravitational fields, their position will shift by a tiny amount. From that, scientists can reconstruct the Moon’s gravity field in far greater detail than ever before.

The GRAIL spacecraft won’t head directly for the Moon, though. Instead, they’ll first head away from the Moon — toward a point in space where the gravity of Earth, Sun, and Moon are in balance. That path actually requires less fuel than a more direct approach to the Moon. It also gives the Sun time to bake out any trace amounts of air or other gases the craft might contain, and it gives flight controllers time to precisely align the approach to the Moon.

The first craft will enter orbit on December 31st. The second will follow just 25 hours later. Then they’ll spend the next three months mapping the Moon’s gravity — by playing a precise game of follow the leader.

Script by Damond Benningfield, Copyright 2011

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

[3]

StarDate is brought to you in part through the generous support of our underwriters. Interested in underwriting? Let us know.